Stainless steel belts are widely used in various fields including, for example, continuous baking of cookies at high temperatures, freezing of foods at low temperatures, continuous pressing of ply wood and transportation of mechanical parts. Since high strength and excellent fatigue strength are required, steel species constituting such belts are generally selected from work hardened austenitic stainless steels, low carbon martensitic stainless steels and precipitation hardened stainless steels.
JP B 51-31085 discloses a low carbon martensitic stainless steel for use in the production of a steel belt comprising by weight from 0.03 to 0.06% of C, 0.5 to 1.0% of Si, not more than 0.3% of N, from 3 to 10% of Ni, from 10 to 18% of Cr, and optionally from 5 to 16 times (C+N) of Ti, the balance being Fe and impurities. It is taught that when a strip of this steel is used in the annealed condition to prepare an endless steel belt by end welding, the endless belt has a high strength in the weld zone and exhibits an extremely reduced deformation due to local heating for the welding.
JP A 61-9903 discloses a process for the production of a steel belt comprising cold rolling a strip of maraging steel or a precipitation hardenable stainless steel to a thickness of 1 mm or less, welding ends of the strip to provide an endless belt, cold working the endless belt at a working rate of from 20 to 50% and aging the cold worked endless belt. The aging is carried out under no tension. It is taught that the endless belt so prepared has satisfactory hardness and tensile strength in the weld zone.
While both the above-mentioned Japanese publications discussed the strength or hardness of the welded endless belt in the weld zone, they make no mention of the surface flatness of the endless belt.
In order to produce a stainless steel belt having a good shape, it will be necessary to prepare a flat stainless steel strip by a process including a rolling step for shape rectification. This step of rolling for shape rectification should be appropriately carried out, while carefully selecting conditions including, for example, rolling reduction, diameters of rolls and rate of rolling, depending upon the steel species, thickness of the strip and histories of the precedent production steps, or otherwise a flat stainless steel strip cannot be obtained or the production yield is reduced. Accordingly, it is eagerly desired to prepare a stainless steel strip excellent in flatness without the rolling step for shape rectification. Unfortunately, the desired technology is not yet established on the concerned steel species.
Even if a flat stainless steel strip has been prepared by rolling for shape rectification, when an endless belt is prepared from the flat strip by end welding, it is unavoidable that a strain due to expansion and contraction of the material is induced near the weld zone as a result of the welding. For this reason, the endless belt so prepared must be again flattened by stretching or by stretching and rolling.
Thus, in order to produce a high strength stainless steel belt in a high production yield, precise and complicated steps have been required. Particularly, when a wide steel belt is to be produced, two or more starting plates are longitudinally welded side by side to provide a wide welded plate, which is then made endless by end welding. In such a case, the flattening of the welded wide endless belt under tension is further complicated, time consuming and technically very difficult.
U.S. Pat. No. 4,878,955 to Hoshino and Igawa discloses and claims a process for preparing a process for preparing a high strength stainless steel material having excellent workability free from weld softening consisting of a single martensitic phase of a duplex phase structure of martensite and minute austenite, the process comprising heat-treating at a temperature of 550.degree. to 675.degree. C. for 1 to 30 hours cold rolled material of a steel wherein no annealing treatment is performed between a final cold rolling step and said heat-treatment step, said steel consisting essentially of:
C: not more than 0.10% PA1 Si: 0.85-4.5% PA1 Mn: 0.20-5.0% PA1 P: not more than 0.060% PA1 S: not more than 0.030% PA1 Cr: 10.0-17.0% PA1 Ni: 3.0-8.0% PA1 N: not more than 0.10% and Fe and inevitable impurities, wherein the Ni.sub.eq value is defined as: PA1 Ni.sub.eq =Ni+Mn+0.5 Cr+0.3 Si+20(C+N) is in the range of 13.0-17.5.
This U.S. patent is, however, completely silent with respect to a stainless steel endless belt and does not deal with a problem of surface flatness of a stainless steel endless belt.